This invention relates to compositions and methods for forming glass seals.
The use of special glass compositions for sealing (sometimes referred to as "soldering") glasses or other related materials, such as ceramics, together is well known. Such glasses of the vitreous type were known to have drawbacks, in that they were often insufficiently strong to withstand the rigors of use, primarily due to the fact that such glasses often have coefficients of thermal expansion much higher than those of the glasses which they are expected to bond together; therefore, upon cooling after heat-sealing, undue stresses may be set up in the glasses, which weakens the seal.
In order to overcome some of the problems associated with vitreous solder glasses, solder glasses were developed having the property of being initially vitreous but of crystallizing (devitrifying) during heat-sealing. Such devitrification or crystallization tends to strengthen the seal structure and lower the coefficient of expansion of the seal.
In addition to the lead oxide, zinc oxide, boron oxide, and other oxides which typically constitute such devitrifiable solder glasses, U.S. Pat. No. 3,258,350, issued June 28, 1966, and U.S. Pat. No. 3,250,631, issued May 10, 1966, describe the addition of other components, such as zircon or .beta.-eucryptite, respectively, in order to better overcome the problem of the coefficient of expansion being higher for the soldering glass than for the materials being joined together.
U.S. Pat. No. 3,778,242 issued Dec. 11, 1973, describes devitrifiable solder glass compositions having favorable properties with respect to the time and temperature factors necessary to forming a seal, and achieves this by utilizing a composition comprising an uncrystallized (vitreous) but crystallizable (devitrifiable) solder glass modified with a refractory material and a pre-crystallized glass.
U.S. Pat. No. 3,954,486, issued May 4, 1976 relates to another modification in sealing glass compositions and methods of using the same, wherein it is stated that improved results could be obtained if the particle size diameter of the refractory filler is in excess of 37 microns and not substantially in excess of 150 microns which is significantly different than in the present invention. The glass of Example 4 of this patent appears similar in composition in some respects to some of the glasses described below, but that glass has been melted in our laboratory and found to be a divitrifying glass, quite different in behavior from the glasses of the present invention. In contrast, the glasses of the present invention are not devitifying compositions that do not contain alkaline oxides and generally do not contain silica, both of which tend to encourage devitrification. U.S. Pat. No. 4,006,028, issued Feb. 1, 1977, discloses a devitrifying sealing glass using a finer .beta.-eucryptite which is still much coarser than that used in the present invention.
U.S. Pat. No. 3,907,585, issued Sept. 23, 1975 describes certain problems that occur using devitrifiable glass and inert refractory particles as described above, and although noting that non-crystallizing glass solders mixed with refractory substances were, upon the basis of preliminary experiments, thought useful to circumvent such problems, states that additional experiments have established that mixtures of noncrystallizing glass solders having a low softening point and .beta.-eucryptite or similar substances are normally unsuitable from a technical standpoint (e.g., because intolerably high mechanical stresses develop between the mixture components). To solve such problems, U.S. Pat. No. 3,907,535 describes a process for soldering using a thermally devitrifiable solder glass and .beta.-eucryptite, wherein the soldering is discontinued before 50% of the total volume of the composition crystallizes. The examples describe soldering operations for such compositions at 450.degree. C. and 550.degree. C., with the specification generally referring to a range of up to 450.degree. C.-600.degree. C.
Another problem that has been assumed to exist with respect to such mixed phase seal-glasses is that they cannot undergo a redraw process; however, IBM Technical Disclosure Bulletin Volume 20, No. 7, December, 1977, indicates that mixed phase seal-glasses can be redrawn with an appropriate choice of redraw temperature. The seal material used by Diel is believed to have been a mixture of .beta.-eucryptite and a glass which unlike the materials claimed herein is a devitrifying glass.
U.S. Pat. No. 3,734,702, issued May 22, 1973, describes a crystallizable fusion-type glass sealing composition in which aluminum titanate is incorporated in a lead borosilicate or lead-zinc-borate solder glass, in order to inhibit crystallization, thus providing more time for forming a seal without developing uneven stress concentrations due to crystallization.
Because of limitations inherent in the properties of certain materials which it may be desired to seal, the devitrifiable solder glasses discussed above may be additionally unsatisfactory due to the fact that the high temperatures associated with the devitrified nature of the solder glasses may result in deterioration of the materials being sealed. For example, in the construction of gas panel displays, a dielectric glass is formed over conductors on a plate glass substrate, followed by application of a magnesium oxide coating over the dielectric, and finally by sealing at a temperature of about 485.degree.-490.degree. C. However, due to the fact that there is a maximum temperature-time combination to which the dielectric can be exposed during sealing, and the fact that magnesium-oxide crazing occurs at temperatures of approximately 495.degree. C., a continuing need for improved sealing glass compositions exist. Desirably, such sealing glass compositions could be used for sealing at temperatures less than 450.degree. C. and would have coefficients of thermal expansion and mechanical properties corresponding to those of the materials being sealed.